Trajectory investigations of the dissociation dynamics of vinyl bromide on an ab initio potential-energy surface

The reaction dynamics of vibrationally excited vinyl bromide have been investigated using classical trajectory methods on a global, analytic potential-energy hypersurface that is developed primarily by least-squares fitting of appropriately chosen functional forms to the results of ab initio electronic structure calculations. These calculations are carried out at the MP4 level of theory with all single, double, and triple excitations included. A 6-31G(d,p) basis set is employed for the carbon and hydrogen atoms. Huzinaga's (4333/433/4) basis set augmented with split outer s and p orbitals (43321/3321/4) and a polarization forbital with an exponent of 0.5 is used for the bromine atom. The present calculations focus upon the determination of the dependence of the potential upon the stretching coordinates for the bonded atoms in vinyl bromide, the C-C-H and C-C-Br bending coordinates and the dihedral angles. The couplings between these coordinates are also investigated. The total ab initio database is obtained by combining these results with previously reported studies of the vinylidene-acetylene system and our previous calculations of saddle-point geometries and energies for the various decomposition channels, reactant and product equilibrium structures, and vibrational frequencies. The analytic surface fitted to this data base (PES 1) is then modified by adjustment of the potential curvatures at equilibrium to provide a better fit to the measured IR and Raman vibrational frequencies of vinyl bromide while simultaneously holding all other topographical features of the surface constant to the maximum extent possible. The surface so developed is labeled PES2. Finally, we have arbitrarily altered the reaction coordinate curvatures for three-center HBr and H-2 elimination to produce a third potential surface (PES3). The dissociation dynamics of vinyl bromide on each of these potential surfaces are investigated at several excitation energies in the range 4.5 to 6.44 eV. Total decomposition rate coefficients and product branching ratios are computed as a function of excitation energy. The HBr vibrational-state distribution is computed and found to be Boltzmann with an effective vibrational temperature of 7084 K on PES1. These results are in virtually exact agreement with recently reported measurements of this distribution. Finally, we have investigated the dissociation mechanisms for three-center Ha and HBr elimination reactions. The results show that the dynamics are very similar on PES 1, PES2, and PES3. Consequently, small variations in potential-energy curvatures at equilibrium and along the reaction coordinates do not exert significant influence upon the dissociation dynamics. However, some large qualitative variations between the dynamics on the ab initio surface and the more empirical surface previously employed are found to exist. We conclude that great care must be exercised when such surfaces are used to study reaction dynamics in polyatomic systems.